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  1. #1
    Join Date
    Mar 2011
    Location
    Maple Grove, MN
    Posts
    7

    Dew point control

    I operate my air conditioning different than everybody else. I'm looking for the best way to keep the humidity/dew point low in the house where it is comfortable but I don't really have a goal of 72 degrees as I'm more comfortable with heat than most people as long as its not very humid in the house at the same time.

    What I've been doing is early in the morning before it gets hot out(but not when its too cold for the a/c to run), I will run the air conditioning for 2 hours and shut it off. Then later on in the day when it gets hot, there is less humidity in the house and it doesn't feel warm. If it gets hot or humid in the house again during the day, I'll run it another two hours right before going to sleep so the house is comfortable.

    I've done a fair amount of sun reduction through growing deciduous trees in the south and west sides of the house and also installed a few layers of bug screens to the sunnier picture windows to reduce the heat from the sun, it's working very well.

    When I run the a/c for two hours in the evening on an 88 degree day where the inside got to 80 degrees, it will drop the temperature to 76 and the humidity to 34% (dew point 46), the basement is 62 degrees and 58% humidity in the same condition. The basement isn't finished and no ductwork to it(not looking to have any due to radon concerns) but it isn't isolated from the rest of the house in any way.

    The equipment is a Carrier 58GS that had a blower motor die last fall and the tech retrofitted it with a PSC motor that consumes less power, he measured the ductwork and used his equipment on the returns and supply vents and says that its putting out 800CFM set to its highest speed tap with a new filter installed. The condenser is a Janitrol with a H21B223ABCA Bristol compressor which my tech says is a 23k compressor and the unit's nameplates are weathered and completely white but he spent the time to do a reverse look up and says he is 90% sure it is a Janitrol 24CL-1AB 8.5 SEER unit. His inspection with poking thermometers in places and measuring pressures and airflow is that the unit already has a proper charge and doesn't need any adjustment.

    I showed him my electric bills and told him how I use air conditioning and he said that it wouldn't make any sense, financially to replace the unit unless the furnace cracks(he checked and its not) or if there was some other expensive repair. So far everything is fine except for the blower motor replacement which wasn't that much. He measured current draw, it is pulling 10 amps when it was 80 degrees out and he said that seemed normal to him.

    When I run the unit for many hours, the humidity is mostly sapped from the air after 1.5 hours and anything longer than that and it seems the temperature drops slowly and the humidity stays steady. The tech said that when the unit shuts off that the coils moisture goes back into the house and that if I went with a 16SEER or higher unit that it would take more time to start removing the humidity and once I shut it off, it would be more humidity that gets added back. He also said the coil itself wouldn't be as cold and the humidity removal less effective for the amount of power used unless I wanted to drop the temperature to the same 72 degrees most people set their house to. This all makes sense but my question is, is 2 hours the right amount of time to gather and remove most of the easily removable latent heat from the air? Should I run it 3 hours or is 2 plenty? I'm really just trying to effectively remove the most humidity from the air as I feel comfortable in the house as long as it doesn't feel muggy.

    House is 2200 sq ft total and the zip code is 55369.
    What do you guys think, is 2 hours about the time it takes to remove the bulk of the humidity in the air? Thanks in advance.

  2. #2
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    It is true that the older 8-10 SEER coils had less surface area so they tend ot remove humidity better. Ooe of the ways mfg increase efficency was to increase coil sizes/surface area to better heat transfer.

    There is a limitation to how much moisture you can remove with a refrigeration based systm. You normally don't run coil temps much below 40F in normaly operation. This is therefore the lowest dewpoint you can achieve.

    Being in MN< humidity won;t be a major issue. You may have some high humidity days in spring and fall when there's little cooling load ot run the AC. But if the equipment is sized right, which it sounds liek it is with your long runtimes, you wil have good humidity number like your showing.

    You cooling season is so short, that I'd just run that equipmet to failure, especially if you just paid ot replace the compressor. Then make sure you get a load calculation done prior ot repalcement and be sure it's size correctly and set-up to dehumidify on demand with a Vairable speed blower. DOD will slwo the blower speed when the humidity is above setpoint to lwoer the coil temprature to remove more moisture and cool the air less.

  3. #3
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,338
    You're fortunate to live in a climate and house that favors your current comfort management strategy.

    If you're comfortable, and you find your electric bills reasonable, don't change anything until your equipment kicks the bucket forever. From a health standpoint, maintaining indoor dew points below 55 degrees is the best assurance against mold and other moisture related problems.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  4. #4
    Join Date
    Mar 2011
    Location
    Maple Grove, MN
    Posts
    7
    motoguy128, It was the blower motor in the furnace that was replaced, not the compressor. When the blower motor was failing I didn't realize it right away and the air flow dropped enough to freeze the coil and the lines all the way back to the condenser. I found out about it when I noticed the register in the kitchen wasn't kicking out the same amount of cold air as usual, went to the basement and there was ice on the outside of the supply ductwork and I immediately powered it off, suction line iced back to the condenser. I just figured it was an odd occurance until it ran the next day and I checked it after it was running for an hour and the blower was on its way to grinding to a halt. It was oiled earlier in the spring but sleeve bearings don't last forever.
    I'd have to disagree with humidity not being an issue, we get a ton of it here, last summer we had 70+ degree dew points with cloudy days, it can be an issue here, just not all the time.

    Shophound, yeah, it's not too hot here, we have two months were highs get into the 80's and usually a weeks worth that touch above 90 but it's not too bad. Electric bills in July operating my equipment as I do and focusing on energy efficiency throughout the house and not just the A/C(including use of efficient lighting, electronics, and R50 attic, R30 for attic knee, and R19 for the rest of the walls, not running the oven often in the summer, and as much air sealing as I could do.) has yielded a $70 electric bill. Keep in
    mind I'm okay with 78 degrees if the humidity is kept low so that really helps.

    Thanks for the 55 degree dew point target, I've been aiming for 50 degrees but sometimes it climbs a few higher between when the A/C runs if I'm not going to be home much during the day.

    I just wish there was a smart thermostat that looks at the forecast of temperature, expected outdoor dew point, and then runs the air conditioning in long cycles to make things more efficient and a little more hands off then just setting a temperature since comfort and efficient equipment operation has more to do with other factors than just temperature. Having a thermostat that factors in the expected outdoor temperatures would be extremely useful in having the equipment recover from setback at just the right time and cool things off before people go to sleep.

    Is there any higher SEER equipment that uses a standard/smaller evaporator coil but uses more efficient motors/compressors and a larger condenser coil? That seems to be where I'd aim for if I wanted to continue what I'm doing now if I was prompted with an equipment failure and had to replace things.

    Also regarding capacity, it seems I've got the right size now because on an unusually hot day where its above design temp(88 I think), if I set the temp to 72 like everyone else does and it gets to be 95 outside with full sun, the a/c will keep the house at about 74 degrees at its hottest and run all day, which I'd expect over design temp. Plenty comfortable for me but if I were to downsize to a 1.5 ton but run 1+ hour cycles would there really be an advantage on an averagish day since its not cycling for a short time period? I also notice that the units beyond 13 SEER frequently don't come in 1.5 ton since that would usually mean there is a low cooling load.

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,064
    Might want to check into a whole house dehumidifier.
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  6. #6
    Join Date
    Mar 2011
    Location
    Maple Grove, MN
    Posts
    7
    I thought about the whole house dehumidifier route, but at the same time, the humidity is there when there is plenty of heat that could be removed as well. I rarely have a need to overcool to keep the basement humidity/dew point in check.

    I have considered a energy or heat recovery ventilator and if I do decide to go for one, I'm looking at those super efficient Ultra-aire units that have the capability of supplying fresh air. If they come with a multi-speed control, I'd crank the ventilation while its cooler outside which I'd expect would take a dent out of the cooling costs while bringing in air and dehumidifying it. Does the Ultra-aire XT105H function as a recovery ventilator with a heat exchanger or does it pull raw outside air in and just dehumidify it?

  7. #7
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,338
    Controlling humidity via dew point vs. relative humidity is slowly catching on in commercial HVAC. At some point someone may make a thermostat for residential that can do the same thing. Problem is, everyone is steeped in understanding air moisture content via relative humdity, it can be challenging helping folks understand the superiority of dew point control vs. %RH control. The advantage of dew point control is that it is independent of dry bulb temperature, which of course affects %RH. For human comfort cooling and heating, the range of dry bulb temperature maintained is fairly steady, but many factors can skew RH readings high or low, causing equipment to run too long to remove moisture, or not run long enough, and in winter over or under humidify.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  8. #8
    Join Date
    Mar 2011
    Location
    Maple Grove, MN
    Posts
    7
    I dreamed up the idea of having some sort of dew point sensor that is wired in series with the thermostat when it hits a certain point and the thermostat is calling for cool that it will trip on and stay in a latch-on state for 2 hours to drop the dew point. That would make things hands off for me but sometimes we have 80 degree sunny days that land with a 55 degree superhumid foggy night which is not a condition that I think my a/c without a low-ambient kit installed should be running in. Designing a control like that is above my head so I'm not going to touch it but it would be great for what I'm trying to do.

    What is the generally considered safe outdoor temp that a system can run at if its too high of a dew point to want to open the windows but its 80 degrees inside from a hot day where I was gone but came back at 2am after getting home from doing other things? I've had my system running at 55 degrees outside before, finding out after the fact that it was that cold out while it was cooling things off but I think that may have been pressing my luck. I'm not sure if the price is high on a low ambient kit or if there is a temperature switch that could be installed on the indoor coil or suction line to shut it down instead of freezing up? Would the price be reasonable(or is something like this even avaialble for an old unit like mine) if I had my tech install something like that when he's here for the pre-winter furnace HE crack check?

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,064
    Juan Madera, this is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise here. Please apply to the AOPC today, thank you.

    You can find the rules for posting and qualifications here.


    Further infractions may result in loss of posting privileges.
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  10. #10
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,432
    Quote Originally Posted by Stein View Post
    I thought about the whole house dehumidifier route, but at the same time, the humidity is there when there is plenty of heat that could be removed as well. I rarely have a need to overcool to keep the basement humidity/dew point in check.

    I have considered a energy or heat recovery ventilator and if I do decide to go for one, I'm looking at those super efficient Ultra-aire units that have the capability of supplying fresh air. If they come with a multi-speed control, I'd crank the ventilation while its cooler outside which I'd expect would take a dent out of the cooling costs while bringing in air and dehumidifying it. Does the Ultra-aire XT105H function as a recovery ventilator with a heat exchanger or does it pull raw outside air in and just dehumidify it?
    The Ultra-Aire XT105 H is whole house dehumidifier with the capability of bringing in fresh air on demand, or an occupancy schedule, or on CO2 levels. Homes need makeup air for the clothes drier, bath fans, and the kitchen exhaust hood.
    Ervs exhaust and provide fresh air through a heat exchanger to transfer moistue and heat to the lessor air stream. Most homes get enough fresh air during cold windy weather and therefore do not benefit much from this feature.
    For fresh air when needed and supplemental dehumidification during low/no load cooling conditions, the UA line of whole house dehus are the straight forward method maintaining the temp/%RH.

    There are many days with no/low cooling loads and highoutdoor dew points during which the dehumidifier provides <50%RH throughout your home. I have been removing 20 gals. of moisture per week to maintain <50%RH in my basement. I have had less than 10 hours of cooling upto now.
    Regards tB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

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